70 



OUTLINES OF GEOLOGY. 



160. The several thicknesses given above are the aver- 

 age of many observations. It rarely happens, in spite of the 

 rule of conformity which is observed in the strata of the 

 same formation, that a constant thickness is maintained 

 throughout ; many of the beds, on the contrary, gradually 

 thin out from a maximum, until they, in some cases, become 

 mere seams. In spite of this, every formation higher than 

 the crystaline rocks of the inferior order, appears to be com- 

 posed of a regular series of strata, which occur, in any given 

 district, in a constant order of succession, and which, even 

 in the most distant parts of the globe, do not vary materially 

 from that order. The widest deviations from absolute regu- 

 larity of succession, are found in the Eocene group, and in 

 the coal measures. In the former, the formations of similar 

 date are : in the London basin, a marly clay, and in the 

 Paris basin, the calcaire grossier. The coal measures may, 

 as we have seen, be found in any part of the medial order, 

 between the carboniferous sandstone and the lower rocks of 

 the submedial. The rules do not apply to the rocks which 

 remain to be treated of, namely, those not stratified. They 

 are confined to no specific place ; each of the orders into 

 which we have divided them, may be found traversing, or 

 even intercalated, in the strata of more than one of the 

 groups, and frequently extending through several orders. 



ROCKS NOT STRATIFIED. 



ORDER I. — GRANITOID. 



161. The type of this order is granite, which is a highly 

 crystaline rock, composed of quartz, felspar, and mica. 

 Each of its constituents may also exist as a rock of the same 

 order. This is not generally admitted in respect to the 



